Coated articles are known in the art. Example coated articles, with low-emissivity coatings on glass substrates, are described in U.S. Patent Document Nos. 2001/0041252, U.S. Pat. Nos. 6,059,909, 5,770,321, 5,800,933, 6,916,408, 5,344,718, 5,514,476, 5,584,902, 5,557,462, 6,802,943, 6,830,817 and 6,686,050, the disclosures of which are all hereby incorporated herein by reference. The coatings of these coated articles are examples of low-emissivity (low-E) coatings that are used for solar control purposes. These coatings block significant amounts of infrared (IR) radiation thereby keeping building/vehicle interiors cooler in hot weather conditions for instance.
Certain low-E coated articles are heat treatable (e.g., thermally temperable), while others are not. A known type of heat treatment in this art is referred to as “tempering.”
Tempered glass is typically from about four to ten times stronger than ordinary (or annealed) non-tempered glass. Unlike annealed glass which can shatter into jagged shards when broken, tempered glass fractures into small, relatively harmless pieces. Thermal tempering of a coated article (i.e., tempering the glass substrate thereof) involves heating the glass substrate of the coated article to a temperature of at least about 580 degrees C., more typically at least about 600 or 620 degrees C. The glass then undergoes a high-pressure cooling procedure known as quenching. During quenching, which lasts just seconds, high pressure air blasts the surface of the glass from an array of nozzles so as to cool the outer surfaces of the glass much more quickly than the center of the glass. As the center of the glass cools, it tries to pull back from the outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, which gives tempered glass its improved strength. Annealed glass will break at about 6,000 pounds per square inch (psi). Meanwhile, tempered glass, according to federal specifications, must have a surface compression of 10,000 psi or more; it generally breaks at about 24,000 psi.
It is known that when a coated article including a glass substrate supporting a low-E coating is thermally tempered, the emissivity and sheet resistance (Rs) of the low-E coating often decrease in coatings which are temperable, due to such tempering. It is said that the IR reflecting silver layer(s) of such low-E coatings, which yield emissive properties, may undergo some recrystallization and crystal perfection during heating, thereby leading to improved carrier mobility and thus reduced resistivity and emissivity following tempering. Lower emissivity (normal and/or hemispherical) and lower sheet resistance (Rs) characteristics are highly desirable in low-E coatings, because they indicate an increased ability to block IR radiation from reaching the interior of a building or vehicle on which the coated article is mounted.
However, non-temperable low-E coatings cannot be subjected to such intense heating as that involved in thermal tempering (at least about 580, 600 or 620 degrees C.) because their silver layer(s) are inadequately protected in many instances, and thus suffer undesirable degradation during thermal tempering. Thus, non-temperable (or non-heat-treatable) low-E coatings often suffer an increase in emissivity in combination with an increase in haze, corrosion, or the like, upon exposure to thermal tempering. Coatings which suffer from a significant increase in emissivity, haze and/or corrosion upon thermal tempering typically are not commercially usable. Thus, it will be appreciated that many low-E coatings are not thermally temperable because they suffer destructive degradation as a result of the high temperatures used in thermal tempering.
Accordingly, it will be appreciated that there exists a need in the art for a technique by which a non-temperable low-E coated article can be treated to reduce its emissivity (normal and/or hemispherical) and/or sheet resistance (Rs).
In certain example embodiments of this invention, a non-temperable low-E coated article is subjected to rapid heat treatment, but only so that the glass of the coated article does not heat to an extent necessary for thermal tempering or heat bending purposes. In certain example embodiments of this invention, at least one flame is utilized to rapidly heat the low-E coating of the low-E coated article, the heat from the flame(s) being sufficient to cause at least one IR reflective layer(s) (e.g., silver based layer(s)) of the coating to undergo at least some recrystallization and/or crystal perfection during such heating. However, the heat from the flame is not sufficient to cause thermal tempering or heat bending of the glass. In other words, the tempering of the glass remains below the range necessary for thermal tempering for example. The result is a low-E coated article which is not thermally tempered, but has a low-E coating which has a reduced emissivity and/or sheet resistance compared to if the rapid heat treatment had not been performed.
In certain example embodiments of this invention, there is provided a method of making a coated article with reduced emissivity and/or sheet resistance (Rs), the method comprising: forming a low-E coating on a glass substrate thereby resulting in a coated article, the low-E coating comprising at least one infrared (IR) reflecting layer (e.g., silver based) sandwiched between at least first and second dielectric layers; and rapid heating the coated article comprising the low-E coating and glass substrate, said rapid heating being performed in a manner such that (a) the emissivity and/or sheet resistance of the low-E coating decrease by at least about 3% due to the rapid heating, and (b) the glass substrate does not exceed a temperature of about 400 degrees C. during the rapid heating.